[derekgas]

The return temp should ideally be around 20 degrees lower than the flow temp, this optimises the secondary heatex, but, as you say, causes more condensation. You misunderstood the part about sizing boilers, the size and control of the hot water is paramount for the hot water, some boilers condense in heating AND hot water modes and other do not, this is important for high hot water usage (more than 2 -3 baths a week), if you do not use enough hot water, then using a boiler which condenses in both modes is not economical, neither is using one that doesnt if you do use a lot of hot water, if you wait an hour for the bath, then it is the wrong boiler for you, they are fully modulating, but have flow rates which are dependent on boiler size, the heating is mostly irrelevent in a normal house, because combis are at least 24kw, and can usually cope easily on the heating side, regular boiler (now pay attention here Neil) are sized from 12kw up to 42kw, these are designed to more suit applications where the hot water needs to run more than one bath simoultaneously for example, or where stored water is a requirement for other reasons, give that some houses may only have 4 radiators, but want a quick filling bath, then a 12kw would be more efficient, as will a 40kw boiler if you have 17 radiators and 2 bathrooms with an ensuite and a 300 litre mains fed cylinder. In summary, combis should be sized, and rated for hot water use, regular boilers should be sized for the heating and the hot water requirements.